Hydraulic systems often include operator controllable fluid actuators such as, for example, hydraulic cylinders. These hydraulic systems may include one or more valves having movable valve elements fluidly connected between a pump and the fluid actuator to control a flow rate and/or direction of pressurized fluid to and from chambers of the hydraulic cylinder. These valves may embody, for example, direct drive valves having a linear motor.
Linear motors typically include a rotor rectilinearly movable relative to a stator. The rotor may be connected to a valve element to affect movement thereof relative to a valve housing. The rotor may include a magnetically permeable armature that is biased by magnetic fluxes generated by the stator. The stator may include an electromagnetic coil and a permanent magnet surrounding the armature. An electromagnetic flux is generated when the electromagnetic coil is energized and, as a result, the electromagnetic flux biases the magnetically permeable armature to either a forward stroke position or a rearward stroke position depending upon the polarity of the electromagnetic flux. Linear motors also typically include a mechanical spring to bias the armature to a predetermined position when the electromagnetic coil of the stator is de-energized.
One such linear motor direct drive valve is disclosed in U.S. Pat. No. 5,787,915 (“the '915 patent”), issued to Byers et al. on Mar. 31, 1998. The '915 patent describes a direct drive valve unit having a linear motor and a valve. The linear motor includes a rectilinearly movable armature biased by a spring and surrounded by an annular permanent magnet and two annular electromagnetic coils. The valve includes a valve element supported in a bore formed within a valve housing. The valve element is directly connected to the armature for movement therewith. The spring mechanically biases the armature to a central position relative to the electromagnetic coils and, in turn, biases the valve element to a central position relative to the valve housing. When the electromagnetic coils are de-energized, the armature and valve will remain in the respective central positions as biased by the spring. When the electromagnetic coils are energized by supplying current thereto, the armature overcomes the spring bias and moves between a forward position and a rearward position.
Although the direct drive valve of the '915 patent may include a linear motor to directly move the valve element, the efficiency of the linear motor may be reduced because the force of the spring bias must be overcome during movement of the valve element in at least one of the forward and rearward positions. Additionally, the spring may increase the occurrence of hydraulic system malfunctions and/or contamination because wear and/or failure of the spring may cause fragments of the spring to wash downstream into other hydraulic system components or may cause the valve element to return to a position other than the central position when the electromagnetic coils are de-energized. Furthermore, the additional spring component may add to the complexity and cost of the direct drive valve.
The disclosed linear motor is directed to overcoming one or more of the problems set forth above.